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Author Topic: ColorMunki Photo Spot Readings: LAB to AdobeRGB  (Read 325 times)

nirpat89

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ColorMunki Photo Spot Readings: LAB to AdobeRGB
« on: October 10, 2017, 10:55:28 AM »

Kind of a basic question, but can't find the info in the literature.  What parameters one should use to convert an LAB reading from a ColorMunki Photo to other color spaces like Adobe RGB or ProPhoto RGB.  i.e. the illuminant temperature of D50, D65 etc and the angle 2 or 10 degrees. 

Thanks.

:Niranjan.
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Doug Gray

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Re: ColorMunki Photo Spot Readings: LAB to AdobeRGB
« Reply #1 on: October 10, 2017, 12:50:43 PM »

Kind of a basic question, but can't find the info in the literature.  What parameters one should use to convert an LAB reading from a ColorMunki Photo to other color spaces like Adobe RGB or ProPhoto RGB.  i.e. the illuminant temperature of D50, D65 etc and the angle 2 or 10 degrees. 

Thanks.

:Niranjan.

Bruce Lindbloom's site is the goto place for this. You can select all the above and even change the defaults. L*a*b* is one where the white point is often overlooked. That happens because the ICC specifies (up through V4) that the profile conversion space, which is in a magnitude constrained Lab, is referenced to D50. In fact even sRGB, Adobe RGB, and many others are also referenced to D50 when going to the common, profile conversion space.

L*a*b* (usually shortened to Lab) is an incomplete specification for a color whereas sRGB is more complete, if gamut constrained.  This is because Lab does not specify a white point. Since that is always D50 with ICC profiles, Lab colors are often stated without a white point. One then assumes that D50 is meant but that is an assumption.

Because the CM is designed for standard ICC color profiles, Lab readings should be assumed to already be adapted to D50. That is also the default for Bruce's site. Converting to/from sRGB you will notice that the reference white point default is D50 while a standard "sRGB" monitor is D65. D50 is what the Lab, X,Y,Z, and xyY values are shown in. And those have been adapted to D50.

http://www.brucelindbloom.com/
« Last Edit: October 10, 2017, 01:00:25 PM by Doug Gray »
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nirpat89

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Re: ColorMunki Photo Spot Readings: LAB to AdobeRGB
« Reply #2 on: October 10, 2017, 08:39:06 PM »

Bruce Lindbloom's site is the goto place for this. You can select all the above and even change the defaults. L*a*b* is one where the white point is often overlooked. That happens because the ICC specifies (up through V4) that the profile conversion space, which is in a magnitude constrained Lab, is referenced to D50. In fact even sRGB, Adobe RGB, and many others are also referenced to D50 when going to the common, profile conversion space.

L*a*b* (usually shortened to Lab) is an incomplete specification for a color whereas sRGB is more complete, if gamut constrained.  This is because Lab does not specify a white point. Since that is always D50 with ICC profiles, Lab colors are often stated without a white point. One then assumes that D50 is meant but that is an assumption.

Because the CM is designed for standard ICC color profiles, Lab readings should be assumed to already be adapted to D50. That is also the default for Bruce's site. Converting to/from sRGB you will notice that the reference white point default is D50 while a standard "sRGB" monitor is D65. D50 is what the Lab, X,Y,Z, and xyY values are shown in. And those have been adapted to D50.

http://www.brucelindbloom.com/

Thanks, Doug.  I learn a lot from you.  I guess when it comes to color management, there are no basic questions.... :)

If I understand correctly, if not perhaps fully, I should go ahead and use D50 for the conversion since that is the standard for most icc profiles.  I was somehow thinking the illuminant temperature is that specified by the device, in this case the ColorMunki Photo, as the light source for their measurements.  In any case, Bruce Lindbloom site is the one I was going to use for the conversion.  Lots of good stuff to learn there as well about color management.  For calculations, the other site I like is the easyRGB where it displays the color in the background which is kind of cool.  Don't know if there are differences in the how two do their conversions.

:Niranjan.
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